The basal ganglia comprise a collection of nuclei that are primarily involved in controlling purposeful movements and appropriate behavior. The striatum represents the major component of the basal ganglia which functions by processing cortical inputs and subsequently regulates activity in the output nuclei (the entopeduncular nucleus and substantia nigra) through two distinct pathways. The direct pathway monosynaptically connects the striatum to the output nuclei, while the indirect pathway influences the output nuclei via the globus pallidus and subthalamic nucleus. It is believed that balanced output between these two pathways is required for normal brain function. Indeed, dysfunction of this circuit is known to underlie the abnormal movements observed in the neurodegenerative disorders, Parkinson's and Huntington's disease. Moreover, malfunction of this circuit has also been implicated in the neurodevelopmental disorders Tourette's syndrome and obsessive compulsive disorder (OCD). Currently, little is known about the molecular mechanisms underlying the formation of the striatal output pathways. Our fate mapping data indicates that the LIM homeodomain transcription factor Islet1 is expressed predominantly in the progenitors of the direct striatal output pathway. This proposal will examine the requirement for Islet1 in the correct formation of the direct striatal output pathways. In Specific Aim 1, we will determine the role of this transcription factor in the striatal progenitors, themselves. Specific Aim 2 will determine the requirement for Islet1 in the correct formation of the reticular thalamus and zona incerta as well as their subsequent role in the correct formation of the striatal output pathways. Finally, our preliminary data implicate PlexinD1-Sema3E signaling downstream of Islet1 in the formation of the direct striatal output pathways. Thus, Specific Aim 3 will address the role of PlexinD1 and Sema3E in the formation of the striatal output pathways. Elucidation of the mechanisms underlying the formation of basal ganglia circuitry may lead to a better understanding of the neuronal alterations in certain basal ganglia disorders as well as provide opportunities to develop better treatments for these conditions. PUBLIC HEALTH RELEVANCE: The telencephalon represents the region of the brain most concerned with cognition and voluntary movement. Specifically, the cerebral cortex controls these processes via the basal ganglia, which refine and select appropriate cortical programs for the desired tasks. The major component of the basal ganglia, the striatum (or caudate-putamen) processes cortical inputs and subsequently regulates the output nuclei of the basal ganglia through two output pathways. The direct pathway connects the striatum monosynaptically to the output nuclei while the indirect pathway is a polysynaptic circuit. It is believed that balanced output between the direct and indirect striatal output pathways is crucial for normal brain function. Indeed, malfunction of these striatal pathways occurs in a number of neurodegenerative disorders such as Parkinson's disease and Huntington's chorea, leading to abnormal movements and in some cases dementia. Moreover, certain neurodevelopmental disorders, such as Tourette's syndrome and obsessive compulsive disorder (OCD) have been suggested to result from malfunction and/or altered development of striatal circuitry. At present, little is known about the development of the mammalian striatum and in particular the formation of the striatal output pathways. This proposal will shed light on the molecular mechanisms that control the formation of direct and indirect pathways and specifically the role of the LIM homeodomain protein Islet1 and its down-stream effector molecules such as PlexinD1 in the formation of these important basal ganglia connections.